Interlayer Interface Properties in 3D-Printable Alkali-Activated Mortar with Varying Print Time Gaps

Automation of construction activities through emerging technologies like 3D-printing of concrete helps in improving the productivity of construction projects. 3D-printing of concrete involves layer-by-layer deposition of material to build the structure. The time gap between the successively printed layers is intrinsic to the process of printing, which could affect the mechanical and durability performance of the 3D-printed structures. Further, the print time gap can vary from several minutes to hours/days based on print speed, size of the structure and print execution plan. Hence, this study investigated the effect of wide range of print time gaps (near 0, 0.5, 1, 2, 4, 6, 12 h) on the interlayer interface properties (strength and water absorption rate) in 3D-printable alkali-activated mortar using concrete demolition waste as aggregates. The region of interface between the two consecutive printed layers was also investigated through bulk electrical resistivity and microstructural investigations such as X-ray computed microtomography, scanning electron microscopy, nanoindentation, and surface profilometry. With increase in the print time gap, the bond strength between the layers decreased due to the increased number of defects at the region of interface and weaker bonding between the layers. The reduction was significant ( > 50 % ) for longer print time gaps (4, 6, and 12 h) while negligible for shorter print time gap (0.5 h). A consequent increase in water absorption rate and decrease in electrical resistivity was observed, indicating a reduction in the durability of the filaments. This work contributes to the fundamental understanding of the interface formed between the layers and helps practicing engineers to develop effective print execution plans ensuring the integrity of the structure.